1. Field of the Invention.
This invention relates to graphite fibers, and more specifically, it relates to a class of improved graphite fibers called exfoliated graphite fibers, a method for making exfoliated graphite fibers and composites of exfoliated fibers.
2. Description of the Prior Art.
Graphite is a form of carbon consisting of layers of atoms. The bonding is relatively strong between atoms of the same layer, and is relatively weak between atoms of different layers. By exposing graphite to an appropriate chemical reagent, which is known as the intercalate, the intercalate goes into the graphite and occupies the spaces between the carbon layers. This process is known as intercalation. The resulting material, known as intercalated graphite or a graphite intercalation compound, consists of carbon layers and intercalate layers stacked on top of one another, ideally in a periodic fashion. For example, the stacking can be of the form C-C-I-C-C-I-C-C-I-C, where C stands for a carbon layer and I stands for an intercalate layer. The number of carbon layers between the nearest pair of intercalate layers is known as the stage, which is designated as 1, 2, 3, 4, etc. The herein above recited example is an example of stage 2.
Exfoliation refers to the instantaneous increase in the dimension generally perpendicular to the carbon layers. This process had been performed in graphite flakes (see U.S. Pat. Nos. 1,181,383 and 3,404,601) and pyrolytic graphite (see U.S. Pat. No. 3,404,061).
Processes for roughening the surface of graphite or carbon fibers by heating in various atmospheres have been known. See U.S. Pat. No. 3,476,703. In addition, composites of exfoliated flake graphite have been known. See U.S. Pat. No. 1,137,373.
In spite of the prior known materials and methods, there remains a need for graphite fiber material which possesses the desired properties of increased fiber diameter, reduced density, high flexibility, mechanical strength in the direction of the fiber axis, a high degree of parallel orientation, a high degree of electrical conductivity and thermal conductivity and improved adhesion properties when in composite form.